fix: add debug logging to looper process callback to diagnose audio routing

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

engine.c

@@ -141,6 +141,13 @@ static int process_callback(jack_nframes_t nframes, void *arg) {
         bool any_recording = false;
         bool any_looping = false;
         
+        // Debug: check if we're receiving audio
+        static int debug_counter = 0;
+        if (debug_counter++ % 50 == 0) {
+            fprintf(stderr, "LOOPER DEBUG: Channel %d audio_in[0] = %f, nframes = %u\n", 
+                    ch, audio_in[ch][0], nframes);
+        }
+        
         for (jack_nframes_t i = 0; i < nframes; i++) {
             // Start with live audio input
             rack_in[i] = audio_in[ch][i];